I have the same situation, with my 4X burner I always copied on the fly, but when I went to a 8X I no longer could. After doing the nero actual speed hack I saw why... the reader (an LG) could not keep up anymore.
The odd thing is with an original DVD5 (using AnyDVD) I have no problem copying on the fly at 8X, the reader can keep up, it's with burned discs that it reads them slowly.
Is this strictly an LG issue? Is there a reader that reads burns as quickly as an original?